Tone detection functionality is needed for many common electronic applications. In telephony, for example, several functions require the detection of Dual-Tone Multi-Frequency (DTMF) signaling, which may be generated by the pressing of keys on a telephone handset. Additional applications related to telephony include the controlling of echo cancellers as well as the detection of dial tones and busy tones. Even another application utilizing tone detection is the detection and negotiation of facsimile (fax) transmissions.
Tone detection tasks are commonly performed by digital signal processors (DSPs), specialized microprocessors with architectures well suited to the fast operational needs of digital signal processing. Many advanced DSPs today include a set of multiply-accumulator units (MAC units) capable of performing multiply-accumulate operations (MAC operations) that calculate A=A+B*C in a single clock cycle. MAC operations are used extensively in digital signal processing to convolve filters, calculate dot products, evaluate polynomials, and the like.